1. Field of the Invention
A method and apparatus of handling uplink information under carrier aggregation in a wireless communication system are provided, and more particularly, to a method and apparatus for handling uplink information including scheduling request (SR), buffer status report (BSR), and power headroom report (PHR) under carrier aggregation in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs). Architecture of the radio interface protocol of the LTE system includes three layers, the Physical Layer (L1), the Data Link Layer (L2), and the Network Layer (L3). A control plane of L3 is a Radio Resource Control (RRC) layer, and L2 is further divided into a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer, and a Medium Access Control (MAC) layer.
According to the UE MAC specification, a scheduling request (SR) is triggered for requesting uplink shared channel (UL-SCH) resource for new transmission when no UL-SCH resource is available in a current transmission time interval (TTI). In detail, when no UL-SCH resource is available and the UE has no valid physical uplink control channel (PUCCH) resource for the SR in the current TTI, the UE MAC layer initiates a Random Access procedure, and cancels all pending SRs. When no UL-SCH resource is available and the UE has a valid PUCCH resource for the SR in the current TTI while the current TTI is not a part of a measurement gap, the UE MAC layer instructs the physical layer to signal the SR on the PUCCH for each TTI. A triggered SR is considered as pending until UL-SCH resource is granted for new transmission.
According to the UE MAC specification, a buffer status reporting procedure is used to provide a serving eNB with information about the amount of data available for transmission in an UL transmission buffer of the UE. A buffer status report (BSR) is triggered when a triggering event occurs, and is categorized by regular BSR, periodic BSR, and padding BSR accordingly. For example, a regular BSR is triggered when UL data, which belongs to a logical channel with higher priority than those for which data already existed in the transmission buffer, arrives at the transmission buffer, when a serving cell change occurs, or when a retransmission BSR timer expires and the UE has data available for transmission. Triggering events which cause the periodic BSR and padding BSR are omitted herein. The regular BSR, the periodic BSR, and the padding BSR are reported by different formats including long BSR, short BSR, and truncated BSR. How to decide the BSR format depends on the number of logical channel groups which have buffered data and the number of padding bits.
When the buffer status reporting procedure determines that at least one BSR has been triggered since the last transmission of a BSR or this is the first time that at least one BSR is triggered, and the UE has UL-SCH resource allocated for new transmission for the current TTI, the UE MAC layer instructs a multiplexing and assembly procedure to generate a BSR MAC control element that is included in a MAC protocol data unit (PDU), and controls associated timers, and thereby the BSR is reported on the UL-SCH resource. However, when the UE has no UL resource allocated for new transmission in the current TTI and a regular BSR has been triggered, an SR is triggered. Note that a BSR is triggered intends that a BSR trigger is generated, and the BSR is reported when UL-SCH resource for new transmission arrives. A triggered BSR (or said a BSR trigger) is considered as pending until it is cancelled by UL-SCH resource for new transmission. When the BSR is reported, all pending BSRs are cancelled.
According to the UE MAC specification, a power headroom reporting procedure is used to provide the serving eNB with information about the difference between the UE maximum transmit power and the estimated power for UL-SCH transmission. A power headroom report (PHR) is triggered when a triggering event occurs, e.g. when a prohibit PHR timer expires and the path loss has changed more than a specific value since the last PHR when UE has UL resources for new transmission, a PHR is triggered. When the UE has UL resources allocated for new transmission in the current TTI and the power headroom reporting procedure determines that a PHR has been triggered since the last transmission of a PHR, the UE MAC layer obtains the value of the power headroom from the physical layer, instructs the multiplexing and assembly procedure to generate a PHR MAC control element based on the value reported by the physical layer, and performs related steps, so that the PHR is reported on the UL-SCH resource. The PHR MAC control element is included in a MAC PDU; 2 bits in the beginning of the PHR MAC control element are reserved bits, and a 6-bit power headroom filed for indicating the power headroom level follows.
Recently, the 3GPP is involved in the further advancements for E-UTRA and proposes an LTE-Advanced (LTE-A) system as an enhancement of the LTE system. Carrier aggregation, where two or more component carriers are aggregated, is introduced into the LTE-A system in order to support wider transmission bandwidths, e.g. up to 100 MHz and for spectrum aggregation. A UE of the LTE-A system utilizes multiple component carriers instead of a single carrier used in the former LTE system, to establish multiple links for simultaneously receiving and transmitting on each component carrier. Up to now, the abovementioned SR, BSR, PHR, and associated procedures under carrier aggregation are not clearly specified in the current MAC specification of the LTE-A system.